Conventional conference systems are usually equipped with a sensitive non-directive microphone to capture speech from a plurality of participants. The wide coverage area may compromise with noise protection, as any shield or casing will reduce the audio capturing characteristics. As conference microphones also usually are movable, other electronic components may be exposed to external noise.
In particular, the increased use of Global System for Mobile Communications (GSM) mobile phones has lead to an increasing problem with disturbing noise in video and telephone conferences. This noise is introduced into the conferencing system as a result of interference with the audio capturing components caused by radio transmission from the GSM mobile phones. The acoustic components in a videoconferencing system consist of one or more microphones capturing the near-end audio, one or more loudspeakers presenting the far-end audio and a general signal processing unit (codec). When the GSM mobile phones induce interference noise to the audio system, the noise will be received as a very annoying and disturbing noise at the far-end side and the speech intelligibility will be severely degraded.
The GSM networks make use of the TDMA (Time Division Multiple Access) technique to be able to squeeze more calls onto one channel by dividing a calling channel into a few “discontinuous” pieces. TDMA has 8 time slots (i.e. transmitting for one eighth of the time) and the length of each time slot is 0.57 ms (⅛*1/217). Thus, a GSM mobile phone in transmitting mode emits short duration radio-frequency pulses at a rate of 217 Hz. FIG. 1a shows the TDMA pulses sent out from the GSM phone during a registration message. Since the audio capturing devices (microphones) are AC-coupled to the codec, the TDMA pulse train will be filtered through a band-pass filter and the shape of TDMA pulse train picked up by the microphone is shown in FIG. 1b. 
FIG. 5a shows a GSM induced interference signal combined with normal background noise. The time intervals between each negative spike are 4.6 ms (1/(217 Hz)) and the intervals between the negative and the positive spikes are 0.57 ms. The negative spikes are related to the start of the TDMA time slots and the positive spikes are related to the end of the time slots. The induced interference signal contains the 217 Hz fundamental and a large number of harmonics that overlap the frequency range of speech, and therefore severely degrade speech intelligibility.
The GSM mobile phones radio-frequency pulses in a number of situations, some of which are listed below.                When turning on, the terminal transmits a “power on”-registration message to the base station.        When turning off, the terminal transmits a “power off”-registration message to the base station tower.        Occasionally or periodically the terminal transmits a re-registration message to the base station tower.        When sending a Short Message Service (SMS) message.        When receiving an SMS message.        Prior to and during an incoming call.        Prior to and during an outgoing call        When crossing a border between coverage areas managed by different Mobile Switching Centres (MSC).Hence, a mobile phone emits sharp pulses in a number of situations, and as the mobile phone turns out to be a common property, GSM noise has turned out to be a big problem in conferences.        